Keith Ashcroft: A simmering anger fuelled by low self-esteem and paranoia

Psychiatrist's View: With each killing he regains power and control which he didn't have before, and his rage dissipates

Derrick Bird was described by neighbours yesterday as a quiet man who kept himself to himself – until something pushed him over the edge. It is a familiar pattern in rampage killings. The perpetrators are often perceived to be very ordinary people, and to lead ordinary lives. They are very rarely seen as psychotic or cranky.

They often have low self-esteem and tend to be paranoid, feeling the world is against them. Their emotional lives are dominated by a growing sense of anger. They don't suddenly decide to go on a killing spree – it is something that grows over a long period. Because they are paranoid they have a sense of resentment against the world that builds and builds and builds.

Then it just takes a trigger – losing a partner, losing a job or suffering a perceived slight – to push them over the edge. They are ticking time bombs because of their anger. It is not just about the event of the shooting, it is about their whole lives.

Over a period, the paranoia may have been building and they may have been losing contact with their community and withdrawing into themselves. Often this will have occurred a long time before. Their paranoia heightens the sense that the whole world is against them, which increases their anger.

To deal with this, they find solace in a fantasy world. Often, because of their make-up, it involves guns and shooting. It is a way of temporarily gaining control over their lives. It is very immature to want a gun in order to have a sense of power and fulfilment. But it is a way of regaining control.

Video: 12 dead in gun rampage

Once the gunman has claimed his first victim, people wonder why he doesn't stop. Why doesn't he realise what he has done and feel guilt and shame and remorse about it?

But it is then that the fantasy becomes reality. The acting out of the fantasy gives a surge of power and control and a sense of self-worth. It doesn't feel bad, it feels quite good – it is reinforcing behaviour. So he wants to do it again and again. He has rage and low self-esteem. With each killing he regains power and control which he didn't have before, and his rage dissipates.

Finally, as he realises the terrible consequences of his actions, he becomes desperate and lost. His fantasies of power and control wouldn't necessarily include suicide but there is no other way out. His suicide becomes the final killing.

Many people have feelings of low self-esteem and may mistrust those around them or even suffer paranoia. But they don't go on a killing spree. What makes the few that do, flip? Access to firearms is one factor. Guns are, fortunately, not easy to get, but if people have lethal means of causing violence close at hand there will be more violence. How many people would be killed if every household had a gun? That, thank goodness, is not the case in this country.

An incident like this should make us question our values. We need to think about our exposure to videos and violence – does it make us immune to the effects? We see images of violence every day, and through repetition they lose their power to shock.

It raises wider issues too. We define ourselves through jobs, power and money. People are so driven they have no other sense of who they are. They can't go to the doctor or the priest, so they take a gun and kill people. It really is shocking.

The writer is an investigative psychologist based in Manchester